What does it mean?
A stored code P042F simply means that the powertrain control module (PCM) has detected a malfunction in the exhaust gas recirculation (EGR) valve control system. This particular code indicates that the EGR valve appears (to the PCM) to be stuck in the closed position. This code is applicable to the linear (or step-down) EGR. Designation A applies to a particular position or stage of the control system. Nitrogen oxide (NOx) is produced as a side effect of gasoline and diesel engine operation.
As part of the modern automotive exhaust emission control system, the EGR is necessary to reduce harmful levels of NOx. As the name implies, the EGR system permits the engine to consume a portion of unburned fuel from the exhaust system at controlled intervals. At the heart of the linear EGR system is an electronically controlled valve. This valve is opened (under certain circumstances) to allow exhaust to re-enter the engine intake.
Input signals from the throttle position sensor (TPS), vehicle speed sensor (VSS), and the crankshaft position sensor (CKP) are used by the PCM to determine when and to what degree the EGR valve should be opened or closed. The linear EGR valve opens in stages which are determined by the degree which the throttle is opened, how much load is placed on the engine, and how fast the vehicle is moving.
Most vehicles equipped with a linear EGR system use data from the manifold air pressure (MAP) sensor and/or the differential pressure feedback EGR (DPFE) sensor to determine whether the EGR valve is in the desired position. Some older models have a pintle position sensor integrated within the EGR valve. The position of the EGR valve pintle is monitored by this sensor, which delivers data directly to the PCM.
If the EGR valve is suspected of being stuck closed, a code P042F will be stored and a malfunction indicator lamp (MIL) may be illuminated. Most vehicles will require multiple ignition cycles (with a failure) before MIL illumination will occur.